Throughout this application various references are referred to within parentheses or with arabic numerals within parenthesis. Full bibliographic citations for these publications referred to by arabic numerals may be found at the end of the specification immediately preceding the claims. The disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains.
Human enteroviruses belonging to the family Picornaviridae are characterized by a single-stranded positive RNA genome. Members of this vital family include poliovirus, echoviruses, coxsackieviruses and rhinoviruses. Among these viruses, poliovirus has been the most extensively studied.
Poliovirus is known to be the causative agent of poliomyelitis, a paralytic disease of the central nervous system. This virus is known to exist in three stable serotypes--1, 2 and 3. For over 25 years, this disease has been controlled by the use of both the Sabin oral live-attenuated vaccine and the Salk inactivated virus vaccine. The Sabin vaccine consists of attenuated virus of each serotype, none of which are capable of causing disease. The strains used to produce the vaccine were created by a combination of extensive in vivo and in vitro passage of each of the three wild-type strains through monkey tissue. Upon oral administration, the live virus contained in the Sabin vaccine replicates in the gut, thereby inducing both systemic and local immunity. The killed virus (Salk) vaccine, which is administered intramuscularly, is limited to inducing systemic immunity.
Although the Sabin vaccine is considered to be a safe and effective protection against poliomyelitis, a small number of recipients have developed vaccine-associated the disease.
In an effort to understand the molecular basis of attenuation and reversion, the nucleotide sequences of cDNAs corresponding to each of the 3 attenuated strains and their wild-type progenitors, were compared [A. Nomoto et al., "Complete Nucleotide Sequence of the Attenuated Sabin 1 Strain Genome", Proc. Natl. Acad. Sci. USA, 79, pp. 5793-97 (1982); G. Stanway et al., "Nucleic Acid Sequence of the Region of the Genome Encoding Capsid Protein VP1 of Neurovirulent and Attenuated Type 3 Polioviruses", Eur. J. Biochem., 135, pp. 529-33 (1983); G. Stanway et al., "Comparison of the Complete Nucleotide Sequences of the Genomes of the Neurovirulent Poliovirus P3/Leon/37 and its Attenuated Sabin Vaccine Derivative P3/Leon 12ab", Proc. Natl. Acad. Sci. USA, 79, pp. 1539-43 (1984); and H. Toyoda et al., "Complete Nucleotide Sequences of All Three Poliovirus Serotype Genomes" J. Mol. Biol., 174 pp. 561-585 (1984)]. The observed differences in nucleotide sequence between each wild-type progenitor and its resultant attenuated strain were then further analyzed to determine their relationship to the phenomenon of attenuation.
In serotype 3, for example, the attenuated strain differed from the wild-type strain by only 10 point mutations [G. Stanway et al., (1984), supra]. Of these differences, only the changes at nucleotide positions 472 and 2034 were thought to be strongly associated with attenuation [D. M. A. Evans et al., "Increased Neurovirulence Associated With A Single Nucleotide Change In A Noncoding Region of the Sabin Type 3 Poliovirus Genome", Nature, 314, pp. 548-50 (1985); G. D. Westrop et al., "Genetic Basis of Attenuation of the Sabin Type 3 Oral Poliovirus Vaccine", J. Virol., 63, pp. 1338-44 (1989)].
Prior to the identification of the nucleotides which are linked to attenuation, it was demonstrated that cDNA synthesized from a viral RNA template ("RNA virus cDNA") could be utilized to produce viable poliovirus following transfection of mammalian cells [V. R. Racaniello et al., "Cloned Poliovirus Complementary DNA Is Infectious In Mammalian Cells", Science, 214, pp. 916-19 (1981)]. Such observations created the possibility of producing improved polio vaccines via genetic engineering techniques. This could be achieved by altering the cDNA around the crucial nucleotides so as to minimize reversion to the wild-type nucleotide, while maintaining structural and functional integrity of the virus.
Despite the discovery that RNA virus cDNA can be used to produce viable virus, it has never been demonstrated that these cDNA are accurate copies of the viral RNA present in wild-type or vaccine virus. Moreover, the use of cDNA sequences to determine which nucleotides are linked to attenuation, may have caused one or more critical sites to have been overlooked. This is because the process used to produce cDNA, namely reverse transcription, is known to be errorprone [I. M. Verma, "Reverse Transcriptase", In The Enzymes. Vol. 14, P. D, Boyer, ed., Academic Press, New York, pp. 87-104 (1981)].
Accordingly, a need still exists for the production of RNA virus cDNAs which are truly complementary to the vaccine virus RNA. Moreover, the use of inaccurate RNA virus cDNAs may result in reduced attenuation, if these cDNAs are ultimately to be used to produce vaccines, such as polio vaccines.
The genome of poliovirus is a single-stranded RNA molecule of plus-sense that is approximately 7500 nucleotides in length. The error frequency associated with replication of single-stranded RNA, as for poliovirus, is especially high compared to that of double-stranded DNA (3). Due to this inherent property, every preparation of poliovirus including the original Sabin (SO) strains must be considered genotypically heterogeneous.
Culture conditions (ie. temperature, cell substrate) as well as the homogeneity of the input virus are likely to influence which genotype predominates during amplication of a poliovirus sample. It is therefore not surprising that authorities who regulate the manufacture of OPVs (ie. FDA and WHO) dictate strict guidelines regarding the production of manufacturing seeds as well as the passage level of the seed represented in vaccine (22, 26). These regulations were put into action as an effort to minimize selection and amplification of less attenuated variant strains.
It has been well documented that the attenuated phenotype of the Sabin 3 strain is less genetically stable than the type 1 and 2 vaccine strains (4, 7, 11). In the past, a new manufacturing seed (RSO) was derived from the original Sabin 3 virus by selecting a plaque produced in Vervet monkey kidney cell monolayers from extracted infectious RNA (19). The isolate was chosen based on increased stability of its sensitivity to grow at 40.3.degree. C. (rct marker) during serial passage as well as increased attenuation in monkeys. The sensitivity of growth at temperatures above 37.degree. (rct marker) is still employed as an in vitro biological test to analyze the quality of vaccine strains (13).
A report by Kohara et al. (9) suggested that an infectious cDNA clone might be used to preserve the constancy and quality of the Sabin 1 seed. It is plausible that a similar approach could also benefit the attenuated type 3 strain. Until recently, the literature contained two cDNA sequences for Sabin 3 which differed at nucleotide positions (17, 21). The divergence between these sequences may be due to the fact that passage derivatives and clonal isolates of Sabin 3 rather than actual vaccine virus were used for making the cDNA clones.